Open TTD is like "We want to keep as close as possible to the original TTD with some bugfixes," whereas TTDPatch is like a massive pick-your-mods system wherein you use a text file full of flags/switches (or, more easily, a program that changes it for you) to choose from a myriad of different changes you want to make or leave in the game. The only downside to it is that it uses the stock AI, which is more retarded than a wet paper bag full of rocks and nails.
Seriously, watching it is funny. You can watch it build, say, a railroad track. It'll come upon a tiny dent in the landscape that it needs to go around. Instead of backing up, turning, then scooting around the side of it, most of the time it will loop all the way back, build a bridge over the track it just layed, loop around again, and end up on the other side.
